Axial, swash plate type piston pumps are well known in the art. In cases where the swash plate angle is fixed with respect to its axis of rotation, fluid delivery from the pump is dependent only upon the angular velocity of the rotating swash plate. Thus, increasing the angular velocity of the swash plate provides a higher delivery rate, and decreasing the angular velocity of the swash plate provides lower delivery rate. In certain applications, such as fluid actuated diesel injection systems, for example, the swash plate is driven directly by the engine and the angular velocity of the swash plate is dependent upon engine speed. As a result, pump delivery is dependent upon engine speed. However, there are many instances in which it is desirable to control pump delivery independent of engine speed or angular velocity of the swash plate.
To allow variable fluid delivery from the pump independent of angular velocity of the swash plate, it is well known to utilize a swash plate that can be moved to various angles relative to its axis of rotation to thereby vary the displacement of the pump. Such a pump is often referred to as a wobble plate pump. Wobble plate pumps provide satisfactory variable delivery for many applications, but they are often mechanically complex and more prone to failure.
So-called "sleeve metered" pumps have been developed to achieve the variable delivery available from wobble plate pumps without a complex mechanical pump structure. An exemplary sleeve metered pump is illustrated in U.S. Pat. No. 5,603,609, granted Feb. 18, 1997, to Kadlico. In general, sleeve metering utilizes a fixed angle swash plate, thus providing a fixed displacement pump. Each piston is provided with a radial vent port in a portion thereof that does not travel within the cylinder so that fluid is vented through the vent port during the compression stroke. A sleeve is slidably disposed around the portion of each piston that does not travel within the cylinder. As the piston moves during the compression stroke, fluid is vented through the vent port of the piston until the vent port moves through the sleeve, which closes the port. When the vent port is closed, fluid is compressed and pumped at high pressure from the cylinder through an outlet port. Thus, moving the sleeve to cover the vent port longer or shorter durations during the compression stroke varies fluid delivery from the pump. Like wobble plate pumps, sleeve metered pumps are also satisfactory for many applications, but they often require complex mechanical systems and controls to move the sleeves relative to the pistons and are thus more prone to failure.
This invention is directed to overcoming one or more of the problems or concerns set forth above.